Fuel cells which generate electric current by controllably combining elemental hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by a permeable electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a “solid oxide fuel cell” (SOFC). Either pure hydrogen or reformate is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode. Each O2 molecule is split and reduced to two O−1 ions at the cathode/electrolyte interface. The oxygen ions diffuse through the electrolyte and combine at the anode/electrolyte interface with four hydrogen ions to form two molecules of water. The anode and the cathode are connected externally through the load to complete the circuit whereby four electrons are transferred from the anode to the cathode. When hydrogen is derived from “reformed” hydrocarbons, the “reformate” gas includes CO which is converted to CO2 at the anode/electrolyte interface. Reformed gasoline is a commonly used fuel in automotive fuel cell applications.
A single cell is capable of generating a relatively small voltage and wattage, typically about 0.7 volts and less than about 2 watts per cm2 of active area. Therefore, in practice it is usual to stack together in electrical series a plurality of cells. Because each anode and cathode must have a free space for passage of gas over its surface, the cells are separated by perimeter spacers which are vented to permit flow of gas to the anodes and cathodes as desired but which form seals on their axial surfaces to prevent gas leakage from the sides of the stack. Adjacent cells are connected electrically by “interconnect” elements in the stack, the interconnect elements typically being unfeatured flat plates formed of a superalloy or stainless steel. The outer surfaces of the anodes and cathodes are electrically connected to their respective interconnects by electrical contacts disposed within the gas-flow space, typically by a metallic foam which is readily gas-permeable or by conductive filaments. The outermost, or end, interconnects of the stack define electrical terminals, or “current collectors,” connected across a load.
For electrochemical reasons well known to those skilled in the art, an SOFC requires an elevated operating temperature, typically 750° C. or greater.
For steric reasons, fuel cells are preferably rectangular in plan view. Typically, gas flows into and out of the cells through a vertical manifold formed by aligned perforations near the edges of the components, the hydrogen flowing from its inlet manifold to its outlet manifold across the anodes in a first direction, and the oxygen flowing from its inlet manifold to its outlet manifold across the cathodes in a second direction. Thus, fuel cells are typically square in horizontal plan. The flat interconnect forms the opposite wall of the passageway for gas flow past either electrode.
One problem encountered in prior art cells fueled by reformate is that hydrogen utilization is relatively low. The flow space for reformate between each anode and its associated interconnect promotes laminar flow of gas across the anode surface. As hydrogen is depleted from the gas stream at the anode surface, it is not readily replaced. Increasing the flow rate through the flow space can decrease laminarity and increase turbulence but at an increase in throughput of unreacted hydrogen. What is needed is a means for increasing turbulence in the flowing reformate without increasing flow rate.
Another problem encountered in prior art fuel cells is that localized high temperatures occur in regions of the anode supporting the highest rates of hydrogen/oxygen reaction. Such high temperatures, especially when unevenly distributed over the anode, can be damaging to the anode. Excess heat is abstracted from the fuel cell by the cooling effect of air passing between the interconnects and the cathodes, but this cooling is generalized over the entire interconnect surface and further depends upon radiative emission of heat from the anode into the interconnect through the hydrogen flow space. What is needed is a means for removing heat directly from the anode in local regions of high heat generation.
Another problem encountered in prior art cells fueled by reformate is that, through non-uniform flow of reformate across the anode surface, local regions of low hydrogen concentration can occur. In these regions, oxygen ions electrically migrating through the electrolyte to the anode are not all consumed in reaction with hydrogen and carbon monoxide. Nickel in the anode can be oxidized by a surfeit of oxygen ions, leading to failure of the fuel cell. What is needed is a means for electrically restricting the flow of oxygen ions through the electrolyte, in any regions desired, to optimize the consumption of hydrogen and carbon monoxide without oxidizing nickel in the anode.
It is an object of the present invention to reduce the formation of localized superheated regions in the anode.
It is a further object of the invention to reduce the number of components in a fuel cell stack by eliminating the need for a filamentous pad between the electrodes and the interconnects.